The use of continuous fiber reinforced metal matrix composite (MMC) materials in structural applications has increased over the last few years as the aerospace industry took advantage of the superior strength to weight ratio of the new materials. New applications found the fiber reinforced composites panels, such as those with 6061 aluminum or AZ91C magnesium as the metallic matrix, to exhibit large hysteresis and residual dimensional changes during thermal cycling. These drawbacks become critical with applications requiring tight dimensional control and are especially unacceptable on precision spacecraft.
Graphite reinforced MMC materials could represent the next generation of high stiffness, low thermal expansion materials for structural applications in dimensionally stable spacecraft. These materials offer many advantages over the resin-matrix composites, viz., higher electrical and thermal conductivity, better radiation resistance and no outgassing. Currently, the 6061 aluminum alloy is one of the primary metals considered as the matrix for graphite reinforced MMC panels. This material has the very desirable combination of high stiffness and low coefficient of thermal expansion (CTE). In addition, the thermal expansion properties and stiffness may be tailored to a particular application by varying the reinforcement fiber type, number of plies, and the ply orientation. Considering the myriad of advantages, it is particularly important to solve the associatied problems.
For spacecraft applications in Earth orbit, the expected maximum temperature range over which the composite must be dimensionally stable is about 250.degree. F. to -250.degree. F. depending upon the thermal control coating and/or shields used. Initial testing of graphite/aluminum (Gr/Al) MMC materials over this temperature range has revealed significant strain hysteresis and residual dimension changes from the thermal cycle. This behavior is attributed to a high residual stress from fabrication and low matrix elastic limit or strength, which combined with temperature changes, result in plastic deformation within the matrix.
Various methods have been employed to reduce thermal strain hysteresis in metal matrix composite panels with less than satisfying results. One method consisted solely of a standard T6 conditioning heat treatment. This method was successful in eliminating the hysteresis in 6061 aluminum reinforced with P50 graphite fibers but was not effective when the 6061 aluminum was reinforced with P100 graphite fibers. (The numeral designates the elastic modulus in millions of pounds per square inch; P50 has a 50 million pound per square inch elastic modulus.) Success with the P50 graphite reinforced material was probably due to the lower average coefficient of thermal expansion of P50 graphite (0.34.times.10.sup.-6 /.degree.F.) than P100 graphite (0.75.times.10.sup.-6 /.degree.F.), which results in significantly lower thermal strains in the matrix for a given temperature change. The associated lower thermal stresses could be more easily accommodated by the matrix without plastic defomation and without any change in the residual stress state. However, stiffness requirements of the spacecraft structures dictate that P100 Gr/6061 Al composite materials be used.
It is important to find a method that can provide dimensional stability and reduce thermal strain hysteresis in metal matrix composite fabricated both by the hot roll bonding and the diffusion bonding methods of manufacture as both fabrication methods are used for material presently
Accordingly, it is an object of the present invention to provide a method of eliminating thermal strain hysteresis in reinforced metal matrix composite materials.
It is another object of the present invention to provide a method of increasing the dimensional stability of fiber reinforced metal matrix composite materials.
It is yet another object of the present invention to provide a method of treatment to reduce thermal strain hysteresis in reinforced metal matrix composite materials that is employed post fabrication.
It is a further object of the present invention to provide a method of treatment of reinforced metal matrix composite materials that increases dimensional stability and can be utilized post fabrication.
It is another object of the present invention to provide a method of post fabrication treatment to reduce thermal strain hysteresis and dimensional instability in graphite reinforced metal matrix composite material that uses P100 graphite fibers.
It is yet another object of the present invention to provide a method of post fabrication treatment to reduce thermal strain hysteresis and provide dimensional stability to metal matrix composite materials fabricated both by hot roll bonding and by diffusion bonding methods.